In mainstream liquid crystal displays, such as thin-film-transistor liquid crystal display (TFT-LCD), the light is linearly polarized to obtain high contrast ratio. The light emitted from the back light unit, such as cold-cathode fluorescent lamp (CCFL) or light emitting diode (LED), are randomly polarized. As a result, a linear polarizer is needed in front of the LCD panel to select a preferred polarization that transmits the preferred polarization and absorbs tie unwanted one. A problem with this prior art configuration is that more than 50% of the incident light is wasted. In order to efficiently increase power utilization, a wide-angle broadband polarization beam splitter (PBS) is commonly used to recycle the light. One of widely used recycling systems consists of at PBS, a diffuser and a reflector, as shown in FIG. 1. Typically the PBS is a reflective linear-polarizing film produced by 3M under the brand name dual brightness enhancement film (DBEF). Instead of absorbing one polarization, the DBEF separates the incoming unpolarized beams into two linear polarizations, TE or TM, and lets one polarization, TM in this example, pass while reflecting the TE light in this example. The TE light is then depolarized by passing the diffuser and redirected back to PBS by the reflector. In some configurations, the reflector is also a diffuser such as ESR film manufactured by 3M. During a cycle, a portion of this depolarized light transmits through the PBS and the other part is reflected. Through many cycles, the brightness is enhanced for more TM light illuminating on LCD. Typically, the final light recycling efficiency is approximately 60-70%. The diffuser in this example plays the role of converting the TE light into unpolarized, or equivalently, the randomly polarized light. Ideally, if the TE is completely converted into TM light in one recycle, approximately 100% efficiency is obtained without considering absorption.
Several different prior art polarization conversions have been disclosed, one is U.S. Pat. No. 6,064,523 issued to Budd which replaces the diffuser with a quarter-wave plate inserted between the PBS and the reflection mirror. A respective 45 degree linear polarized beam passing through the quarter-wave plate results a right hand (RH) circular polarized wave. For a 90 degree polarization rotation to convert TE light into TM light, the reflected wave needs to be left hand (LR) circular polarized before re-entering the quarter-wave plate. Depending on the relative phase shifts introduced by the reflection mirrors, different mirrors need different arraignments of optical parts to produce LH wave. Two different configurations are shown in FIGS. 2 and 3 as examples. FIG. 2 shows a well known prior all configuration that uses a flat mirror directly reflecting RE circular polarized to LH circular polarized at normal incidence. FIG. 3 is a side view of the prior art structure of parabolic mirror and quarter-wave plate polarization recycle the liquid crystal displays as described in the Budd patent which works equally ell by double reflection with proper mirror-coating. The Budd patent provides detailed explanation of phase formula of parabolic mirror in the different embodiments.
In the above polarization conversion systems, the roles of each of the PBS, quarter-wave plate and reflector are respectively separating beams of different polarizations by the PBS, converting beam polarization is achieved by the Quarter-wave plate and; the reflector redirects the phase-shift beam back to the PBS. With application to direct-view LCD, this approach has three shortcomings. First, the incident angle to the quarter-wave film is usually not normal, thus, the outgoing light is not linear polarized, which limits the conversion efficiency. Second, the backlight is a broadband white light which means a broadband quarter-wave film is needed and its cost is increased. And third, the several recycles do not greatly increase the efficiency due to absorption loss and ineffective polarization conversion.
In tie present invention, the polarization conversion is achieved by the implementation of metallic or metallic-coated grating instead of using the combined quarter-wave plate and reflector, as shown in FIG. 4. More specifically, the metallic grating itself serves not only as a reflector, but also as a polarization converter. It rotates the incoming linear polarization light into elliptic polarized wave. The roles in the current reflective-grating polarization converter including a PBS for separating beams different polarizations and a metallic grating for redirecting, the beam back to PBS while converting the polarization of the beam.
Some metals that exhibit high reflectivity often can be used as broadband reflector. Thus, the redirection of light can be achieved by use of metal or metal-coated material in the grating-based polarization converter. The issue of how to rotate the beam polarization is addressed as following. Polarization conversion by metallic surface grating was experimentally observed, by G. P. Bryan-Brown and J. R. Sambles and published in Journal of Modern Optics, vol. 37, No. 7, P. 1227-1232 (1990). Further, a particular metallic grating structure was reported as broadband polarization-converting mirror in the visible spectral region by I. R. Hooper and J. R. Sambles in Optics Letter, vol. 27, No. 24, pp. 2152-2154 (2002).
In the earlier studies of metallic grating as polarization converter, the incident angle of the light beam was not the main concern in the scientific literature. However, to apply the metallic grating for enhancing the brightness of LCDs, the dependence of the incident angle needed to be rigorously explored because the rays emitted from the backlight unit of the LCD are propagating in all directions. In the present invention, a metallic grating is combined with the PBS to form a new type of resonator, called Polarization Rotation Resonator (PRR). The feature of PRR is a polarization converter with applicability to both broadband and wide-angle incident beams. Its polarization conversion efficiency is not too sensitive to tie incident angle and wavelength.
In the present invention, the insensitivity to angle and wavelength for efficient polarization conversion actually relies on the multi-bouncing of the beam inside the resonator. At each bouncing when the beam hits the metallic grating, not only is the beam reflected, but the polarization is converted too. As a result, each time a portion of the light is transmitted by the PBS and the total conversion is greatly improved even in a few bounces. We can then design the metallic grating to diffract light inside the PRR for several repetitive bouncing to get high conversion efficiency by adding all the converted rays out of PBS.
This structural configuration is similar to Fabry-Perot resonator where the total transmittance is obtained by adding up tall the rays bouncing out of the resonator. At the major region of wavelength, and incident angle, the polarization conversion is approximately 60% at the First bouncing and the overall conversion efficiency can reach above approximately 85%, as shown in FIGS. 11 and 14. Alternative types of diffraction grating with high reflection can be used as long as the polarization conversion is less sensitive to the beam wavelength and propagation direction. With such a nice characteristic, our PRR can actually serve as a broadband and wide-angle polarization converter and since the incident angle and wavelength are not necessarily limited to particular regions, in an embodiment, the resonator structure is replaced by a waveguide allowing the present invention to be used in alternative application where polarization conversion is required.